package com.sloan.community.util;

public class SnowflakeIdGenerator {
    // 设置起始时间戳，如：2021-01-01 00:00:00
    private static final long EPOCH = 1609459200000L;
    private static final long WORKER_ID_BITS = 5L;
    private static final long SEQUENCE_BITS = 12L;
    private static final long MAX_WORKER_ID = ~(-1L << WORKER_ID_BITS);
    private static final long MAX_SEQUENCE = ~(-1L << SEQUENCE_BITS);

    private final long workerId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;

    public SnowflakeIdGenerator(long workerId) {
        if (workerId < 0 || workerId > MAX_WORKER_ID) {
            throw new IllegalArgumentException("Worker ID must be between 0 and " + MAX_WORKER_ID);
        }
        this.workerId = workerId;
    }

    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate ID for " + (lastTimestamp - timestamp) + " milliseconds.");
        }

        if (timestamp == lastTimestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - EPOCH) << (WORKER_ID_BITS + SEQUENCE_BITS))
                | (workerId << SEQUENCE_BITS)
                | sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}

